flattenOffset

和setOffset方法一样，flattenOffset方法也是Aniamted.Value和Aniamted.ValueXY共有的。这个方法也是主要用于处理PanResponder，在方法onPanResponderRelease中调用。

方法flattenOffset会接受offset值，之后用这个值作为基准值（偏差值）加上Aniamted.Value的当前值作为Animated.Value的值，之后把offset值重置为0.比如，现在有offset值100，Animated.Value值为50，当我们调用flattenOffset方法的时候，offset值会被重置为0，而Animated.Value的值为150.

  componentWillMount() {
    this._animatedValue = new Animated.ValueXY()
    this._value = {x: 0, y: 0}

    this._animatedValue.addListener((value) => this._value = value);
    this._panResponder = PanResponder.create({
      onMoveShouldSetResponderCapture: () => true, //Tell iOS that we are allowing the movement
      onMoveShouldSetPanResponderCapture: () => true, // Same here, tell iOS that we allow dragging
      onPanResponderGrant: (e, gestureState) => {
        this._animatedValue.setOffset({x: this._value.x, y: this._value.y});
        this._animatedValue.setValue({x: 0, y: 0});
      },
      onPanResponderMove: Animated.event([
        null, {dx: this._animatedValue.x, dy: this._animatedValue.y}
      ]), // Creates a function to handle the movement and set offsets
      onPanResponderRelease: () => {
        this._animatedValue.flattenOffset(); // Flatten the offset so it resets the default positioning
      }
    });
  },


// To use

<Animated.View style={[styles.box, {transform: this._animatedValue.getTranslateTransform()}]} {...this._panResponder.panHandlers} />